In a scroll device one scroll member orbits with respect to a second scroll member which is typically fixed. Each scroll member has a flat plate or floor portion and an axially extending wrap of a spiral configuration. Ideally, the tips of the wraps of each scroll coact with the floor of the other scroll and the flanks of the wraps of the scrolls coact with each other to define a plurality of trapped volumes or chambers in the shape of lunettes. The lunettes are each approximately 360.degree. in extent and are generally symmetrical but are asymmetrical with respect to the axis of the fixed scroll. The ends of the lunettes, which are defined by the points of tangency or contact between the flanks, are transient in that they are continuously moving towards the center of the wraps as the trapped volumes or chambers continue to reduce in size until they are exposed to the outlet port.
During the compression process, a number of forces come into effect. The gas being compressed acts against the scroll members tending to separate them both radially and axially but because one scroll member is fixed, any movement is limited to the orbiting scroll. Since the axis of the orbiting scroll is located eccentrically with respect to the axis of rotation of the crankshaft, the trapped volumes or chambers are located eccentrically with respect to the axis of the fixed scroll as are the forces associated therewith. Also, there are inertia and friction forces inherent in the driving of the orbiting scroll. To offset these forces a fluid pressure bias has been applied to the back side of the orbiting scroll to offset the axial component of the gas forces, with the net force being the clamping or reaction force, and the bearing supporting the hub of the orbiting scroll has been located so as to minimize the turning moment of the tangential component of the gas forces.
Because leakage must be minimized to have an acceptable device, the fluid pressure bias applied to the back side of the orbiting scroll must exceed the opposing forces so that the plate of the orbiting scroll is held in engagement with the opposing structure of the fixed scroll by a positive clamping force. The excess clamping or reaction force needed to maintain the desired sealing over the entire operating envelope and the friction forces resulting therefrom puts an extra load on the motor and accelerates wear.